(1) Field of the Invention
The present invention relates to a sheet-conveying device which holds a sheet-like material between rotating members to convey it. The invention is directed in particular to a sheet-conveying device in which a rotatable pressing member is provided to press a rotating roller so that a sheet is held and conveyed between the pressing members.
(2) Description of the Prior Art
A typical image recording apparatus is constructed such that; a toner image formed on a recording medium is transferred onto a sheet-like material, a plain paper sheet, for instance, then the toner image held on the copy sheet is fixed thereto and the toner-fixed sheet is discharged from the apparatus. To achieve this sequence, a sheet conveying path is formed along which a sheet-like material is fed to the image forming station and the image-formed sheet is conveyed through the fixing portion and discharged and there are some sheet-like material conveying devices in the sheet conveying path.
Here, one of means for conveying sheets is constructed of a rotating roller and a driven roller, and the driven roller is pressed against the rotating roller so that a sheet: delivered to the nip of the rollers is held and conveyed therebetween. Another means for conveying sheets is provided as a fixing means which comprises a heat roller for heating and fixing a toner image transferred to a sheet and a pressing roller pressing the heat roller for pressing the sheet bearing the aforementioned toner image and fixes the toner image while conveying the sheet.
Other than those described above, a photoreceptor as a recording medium and a transfer roller serve as a sheet conveying means. In some configurations, a transfer roller is provided for pressing the sheet against the surface of the photoreceptor with a toner image formed thereon in order to increase the transfer efficiency of toner image. That is, the transfer roller and the rotating photoreceptor hold the sheet therebetween and transfer the toner image to the sheet while conveying the sheet.
In this case, the sheet is held between and conveyed by the photoreceptor and the transfer roller while the toner image is transferred to the sheet surface. Since the transfer roller is pressed against the photoreceptor, there is a fear that toner particles on the photoreceptor might adhere to the transfer roller. That is, when a sheet to be conveyed is present between the transfer roller and the photoreceptor, toner is transferred to the sheet and no toner transfers to the transfer roller. When a leading end or rear end of the sheet passes through the nip, the photoreceptor and the transfer roller come in pressing contact with each other, and some toner transfers to the transfer roller. Alternatively, if a sheet jams as the sheet is conveyed, the toner image formed on the photoreceptor comes in direct contact with the transfer roller without any sheet therebetween and toner adheres to the transfer roller at the time.
To avoid such adherence of toner to the transfer roller, in some configurations, the transfer roller is provided so that the roller may press the photoreceptor only when it is needed and is kept away from the photoreceptor when it is unneeded. Japanese Patent Application Laid-Open Hei 4 No.204,978 discloses a mechanism for causing a transfer roller to abut against and separate from the photoreceptor on demand. This contact/separation mechanism allows the transfer roller to press the photoreceptor during the image forming and separate from the photoreceptor when no image forming is effected, in order to inhibit toner from adhering to the transfer roller. Besides, toner having adhered to is cleaned by a cleaning brush which is always abutted against the transfer roller.
Japanese Patent Application Laid-Open Hei 4 No.121,767 discloses a transfer belt of belt charging type. This transfer belt is to obtain a good printed image without polluting the transfer belt. To achieve this, semiconductive section is provided at least on one side of the transfer belt. During the transfer operation, the belt is charged so that the static electricity attracts the belt to press the image supporting medium. When no transfer operation is effected, the belt is grounded by a grounding means so that the static charges on the semiconductive section are discharged to the ground. As charges flow away, the static force having pressed the sheet goes out and the transfer belt moves away from the image supporting medium.
In order to reduce the stretch or extension of an endless belt in a printer, it is disclosed in Japanese Patent Application Laid-Open Hei 3 No.179,476 that a tension roller movable up and down is controlled by using a cam mechanism to regulate the tension of the endless belt.
In the sheet conveying means in the form of a heat-fixing device, the pressing roller is adapted to press the heat roller all the time in order to fuse and fix the toner image onto the sheet. Hence, the surface of the heat roller is composed of a relatively soft elastic material such as a silicone rubber and the like to secure a proper contact width (or nip width) with the heat roller. Since the pressing roller is pressed against the heat roller all the time, the surface of the pressing roller tends to yield or deform due to the heat when the pressing roller does not rotate or when no image forming is effected. To deal with this, a proposal has been made in which the pressing roller is made to separate from the heat roller in the non-image-forming mode. Specifically, in the description of the prior art of a microfilm disclosed in Japanese Utility Model Application Laid-Open Sho 57 No.119,351, a fixing means was illustrated in which a pressing roller is made to press a fixing roller (heat roller) by a pressing means made of springs and a solenoid during only the fixing operation.
Thus, according to the prior art, in either of the sheet conveying means made of a photoreceptor and a transfer roller or belt or the sheet conveying means made of a heat roller and a pressing roller, a pressing member pressing a rotating body is provided in such a manner as to press the rotational body or separate from the body as required. For the pressing member to press or separate from the rotating body, the pressing member as well as its supporting member is constructed in such a way as to integrally move to and from the rotating body. This requires a separate means for controlling the movement between the contacted state and the separated state. As a result, the shifting mechanism etc. becomes complicated and also requires increased cost.